【作者】 任丽文；

【导师】 王晓钢； 肖池阶；

【作者基本信息】 大连理工大学 ， 等离子体物理， 2009， 博士

【摘要】 本文重点研究了太阳与空间物理中两个与等离子体波动相关的具体问题:基于Cluster卫星观测数据的地磁尾磁场重联过程中低杂波提供的反常电阻的研究和日冕中电流片内冕环协同加热机制的研究。磁场重联是空间、天体以及实验室等离子体中的一种快速的释放能量的方式,例如太阳耀斑,日冕物质抛射,磁暴和磁层亚暴,以及托克马克约束装置中的锯齿模不稳定性等。关于磁场重联的研究一直以来就被大家所重视,而其中理想磁流体力学的磁冻结效应的破坏是磁重联理论最关注的中心问题。重联点的磁冻结效应的破坏为磁场重联提供了驱动机制。在这个问题上,反常电阻一直以来备受关注,被认为可以为快速磁重联提供有效的耗散机制。尤其是低杂漂移波不稳定性更被许多学者认为是提供反常电阻的首选。一直以来,不论是理论还是数值模拟都表明出现大的密度梯度和磁场梯度的地方,抗磁电流往往会激发低杂漂移波不稳定性,但是这些梯度一般分布在等离子体电流片的边缘,而真正需要反常电阻来激发磁重联的地方是在磁重联发生的电流片中心。也有一些研究者研究过重联过程中的低杂波以及与其相关的反常电阻,然而,由于他们的观测位置距重联点较远,得到的结果是“负的”,即低杂波提供的反常电阻远不能满足快磁重联所需。本文中,我们报道一则由Cluster卫星穿越电流片中心的磁尾重联事例。根据电场在低混杂频率附近能量谱求得相应的反常电阻,并将之与由卫星数据直接求得的重联发生所需的有效电阻进行比较。结果发现在电流片中心,X-点附近,低杂波频率湍流产生的反常电阻可以触发快速磁重联;而在磁场重联的出流区内电流片的中心（B_x=0）,得到了与以往工作[Bale et al（2002）;Carter et al（2002）]相同的“负的”结果,即低混杂漂移波产生反常电阻远远小于重联发生实际所需电阻。第二部分工作主要介绍日冕电流片中冕环的协同加热机制。日冕加热作为太阳物理的挑战性的问题之一,长达几十年来一直困扰着科学家。对于太阳表面（光球层、色球层）温度只有6000度左右的太阳,其外层大气——日冕的温度竟高达百万度,并因此导致巨大的能量以辐射、扩散、传导等形式从日冕流向太空中(在日冕活动区其能流密度高达～10⁴Wm^-2,即使宁静区强度也有3×10²Wm^-2)。一定存在一种加热机制使得日冕维持如此高的温度,平衡因为各种耗散机制引起的能量损失。基于此,科学家们提出了各种各样的加热机制。基于足点的运动时间尺度和剪切Alfvén波的特征传播频率的比较,这些加热机制大致可分为:由J·E≈ηJ²引起的“直流”（DC）欧姆加热（或称焦耳加热）以及等离子体波——粒子相互作用引起的“交流”（AC）波加热两大类。对于在日冕等离子体子中究竟是电流片欧姆加热还是波加热,这一直是许多科学家致力研究的问题。但实际上我们认为在日冕电流片中两种加热机制相互关联、相互补充、相互影响的。因此,日冕加热过程问题很可能是一种与磁场拓扑位型紧密相关的协同加热（SyntheticHeating）过程。本文中,我们提出了一个奇异层电流片中欧姆加热和波加热相结合的协同加热机制。我们知道,磁场重联过程中会形成小尺度的奇异的电流片结构（这是日冕电流片加热理论提出的一种主要机制）,冕环足点的对流运动也可以在磁分形面产生小尺度的电流片结构,同时这个小尺度的奇异电流片也可以由剪切流导致的Alfvén共振引发。与此同时,在奇异的电流片结构这样非常小的空间尺度上,必须考虑到双流体效应,从而可以激发动力学Alfvén波（Kinetic Alfvén Wave,KAW）。而动力学Alfvén波也是日冕波加热理论提出的一种主要机制。我们大致估算了存在奇异层电流片时两种加热机制的加热功率,可以看出,在小尺度的电流结构中,不仅存在欧姆加热,而且也存在波的加热,二者共同对加热日冕等离子体做出贡献。这样一个加热机制也许可以改善我们对日冕加热问题的理解。基于以上,本论文的安排如下,第一章,综述了本文课题的研究背景。介绍了基础物理知识,日冕物理,地磁层内磁场重联,以及涉及到的等离子体波动的基本内容;第二章,介绍了地磁尾重联观测问题中所用到的卫星,数据,及数据处理方法;第三章,重点在低杂波频段的湍流提供的反常电阻激发重联的问题研究;第四章,日冕加热问题的研究,在现有理论的基础上,提出电流片中日冕的协同加热,即欧姆加热和KAW波加热两种加热机制共同加热日冕等离子体。最后,我们对整篇论文进行了一个简要的总结。更多还原

【Abstract】 This dissertation focuses on two important problems related to plasma waves in solar and space plasmas: studies for anomalous resistivity due to lower hybrid frequency turbulence in fast magnetic reconnection in magnetotail based on Cluster satellite data and the synthetic coronal heating mechanism on current sheets.The violation to the frozen-in condition in ideal magnetohydrodynamics (MHD) is fundamental for magnetic reconnection which is thought decisively important for fast energy release and conversion events in space, astrophysical and laboratory plasmas, such as solar flares, coronal mass ejections, magnetospheric storms and substorms, as well as sawtooth and disruptive instabilities in toroidal confined devices. Magnetic reconnection relies on the dissipation mechanism in a localized narrow zone called "diffusion" region, where the frozen-in condition is broken. Resistivity anomalously generated via the wave-particle interaction is thought to be able to provide sizable dissipation to explain violation of frozen-in condition and the sudden onset of fast magnetic reconnection.In this thesis, an intensive spectrum of turbulence peaked at lower-hybrid (LH) frequency near the X-point in a fast magnetic reconnection event is observed by the Cluster spacecraft in the magnetotail. The electric field power density around LH frequency is calculated in the thesis from observed spectrum to measure the resistivity anomalously generated by wave-particle interaction. It is found that, the turbulence at LH frequency induced anomalous resistivity is sufficient to trigger fast reconnection.On the other hand, one of the outstanding problems in solar physics is how to understand the multi-million degree temperature of the solar corona, and correspondingly the large energy flux lost from the corona on the order of～10~4Wm~2 in the active, and about 3×10~2 Wm~2 in the quiet regions. Thus, there should be a heating source to maintain the high temperature of the corona and balance the energy loss due to radiation, thermal diffusion and convection, as well as other ways. Based on whether the timescale of the motion of the footpoints is longer or shorter than the shear Alfvén transit time along the loop, the proposed heating mechanisms can be divided into two groups: direct current (DC) and alternating current (AC) heating. The DC heating is the dissipation of the magnetic energy in conventional Joule heating process. On the other hand, the AC heating, or wave heating, is thought to be caused by wave energy dissipation.In this dissertation, a synthetic heating model of solar coronal loops combining current sheet and wave heating mechanisms is proposed. The formation of singular current structures such as current sheets can be caused not only by magnetic reconnection and footpoint convection of coronal loops, but also Alfvén resonance induced by shear flows. On the other hand, with the Hall magnetohydrodynamics (MHD) effect, the kinetic Alfvén wave (KAW) can also be excited on such current structures. Therefore a synthetic energy dispassion process of both current and wave heating mechanisms on the current structures may lead to better understanding of the coronal heating problem.The contents of this thesis are arranged as follows:In Chapter 1, basic theory and physical problem of reconnection, coronal physics, and related plasma waves are introduced briefly; Chapter 2 describes the data from Cluster, and the related analysis methods used in this thesis; Chapter 3 presents the observation of the anomalous resistivity due to lower hybrid frequency turbulence in magnetotail during a reconnection event; In Chapter 4, a synthetic coronal heating model on current sheets is proposed. Then, a brief summary concludes the thesis in the last chapter.更多还原